Active pixel sensors are well known in the art. A basic description of the active pixel sensors found in U.S. Pat. No. 5,471,515, the disclosure of which is incorporated by reference to the extent necessary for proper understanding.
An active pixel sensor, and many other image sensors, have inherent trade-offs. Typically, the trade-off is made between sensitivity, versus motion resolution, versus space resolution.
For example, we obtain sensitivity by increasing the integration time. However, with a higher integration time, motion becomes more choppy, and hence motion sensitivity is decreased. Sensitivity can also be increased by increasing the pixel size. However, space resolution then decreases, again supporting the trade-off.
Integrated circuit designers continually attempt to put more circuitry on a chip. Lines on the chip are becoming smaller: for example, current technology may use a 0.11 micron process for digital circuitry. However, the image sensor, which is effectively analog, may be subject to a physical minimum size. A pixels that has too small a size and/or high gain, would have insufficient capacitance to allow the sensor to obtain the signal to noise ratio required for quality image acquisition.